Image hard copy mainly relates to screening plate-making technology for a printer and a top-grade plate-making apparatus for printing. Screening technology used for image hard copy is referred to as digital image halftone technology as well. The digital image halftone technology may be classified into two categories, which are amplitude modulation screening and frequency modulation screening respectively. The amplitude modulation screening is referred to as clustered-dot ordered dithering technology as well, characterized in that two geometrically neighboring dyeing dots in a halftone image generated are clustered, thereby forming tufts of dyeing regions, which are called screen dots as well. Because the clustered-dot ordered dithering technology adopts a method of controlling a screen dot area to reproduce grays of an original image, these screen dots are referred to as amplitude modulation screen dots.
Nowadays, in traditional printing technology, the most versatile and widely used technology is amplitude modulation halftone based digital screening technology, and the output amplitude modulation screen dots have different screen dot sizes and arrangement angles, i.e., so-called screen meshes and screen angles, according to an actual use requirement. The screen dot sizes and screen meshes decide clustering degree of screen dots in a one-bit dot matrix, and in theory, the larger the number of screen meshes is, the smaller the screen dot size is; the smaller the number of screen meshes is, the larger the screen dot size is. In terms of a traditional printing output device, the output device generally has a higher accuracy-resolution, which is generally 2400 dpi or more, in order to guarantee output quality of printing and avoid a hierarchical tone sharpening problem based on a one-bit apparatus.
Compared with the traditional printing, new digital printing modes such as digital printing or the like appear in recent years, and have an operating feature very similar to an originally existing desktop publishing system based on printer output. The aims of the digital printing are output quality at a high resolution of the traditional printing and an effect of precise-reproduction of colors. Meanwhile, in connection with emergence of process idea, the demand for the digital printing to be compatible with the one-bit dot matrix output from the traditional printing also gradually emerges, thereby achieving seamless connection between the digital printing and the traditional printing finally.
The digital printing differs from the traditional printing mainly in imaging mode and output precision. Its device imaging mode is mostly ink-jet imaging or carbon powder imaging by a laser, and has an essential difference from color generation based on halftone screen dots in the traditional printing. Meanwhile, the output precision of the digital printing device belongs to low precision output in terms of efficiency, cost and the like, and 300 dpi, 600 dpi, 720 dpi and the like are generally used.
At present, as the digital printing combines with the traditional printing more and more tightly, an image processed by the digital printing not only includes a common eight-bit image, but also increasingly presents a one-bit dot matrix at a high resolution. This brings many technology processing problems, which embody mainly in: how to smoothly convert a one-bit dot matrix at a high resolution to an eight-bit image at a low resolution; how to obtain a converted eight-bit image with information about screen dots, which is easy for the digital printing to make color correction on; and the like. Wherein, the problem of how to obtain a converted eight-bit image with information about screen dots, which is easy for the digital printing to make color correction on, increasingly becomes an insurmountable obstacle in the digital printing process.
In accordance with the above-described problems, they mainly embody in that an eight-bit image, obtained after a one-bit amplitude modulation screen dot image at a high resolution is subjected to conversion for reducing resolution and smoothing processing, is not an eight-bit continuous-tone image in a full sense, chiefly because the converted eight-bit image has information about screen dots in the original one-bit dot matrix, which mainly embodies in screen dots clustered by a plurality of pure-color blocks, and an excess of the amount of values of pure-color pixels in the original one-bit dot matrix destroys tone continuity of an ideal eight-bit image, and seriously hinders subsequent color calibration based on an eight-bit continuous-tone image.